Biomedical Engineering Reference
In-Depth Information
Sol-gel techniques, discussed in Chapter 14
by Risbud and Bartl, are used for preparing
ARCs on transparent surfaces
[39-42]
. During
synthesis, the refractive index of sols can be con-
trolled; for example, titania sol-gels can be made
to have refractive indices as high as 1.8 that are
useful for ARCs on materials such as indium tin
oxide (ITO). Microstructure can be imprinted
onto sol-gels or polymers to enhance their
antireflective properties
[43]
.
There are limitations with index-matching
and quarter-wavelength coatings. First, there is
a theoretical limitation on how much a single-
layer coating can reduce reflection. This can be
overcome by adding layers of coatings; however,
high fabrication cost and limited material selec-
tion are major concerns for multilayer ARCs.
Second, quarter-wavelength coatings are both
narrowband and have a narrow field of vision.
The coatings are narrowband because of their
fixed thickness and as a result they are selective
of the wavelength of light where reflection is
suppressed. Also, the incident rays must be at or
very close to normal incidence for the destruc-
tive interference to occur
[44]
.
FIGURE 12.1
SEM images of a moth eye.
12.1.2 Bioinspired Moth-Eye Broadband
Antireflection Coatings
Solutions to these shortcomings can be found in
nature. Nocturnal moths possess eyes that have
a microstructured cornea that exhibit excellent
broadband antireflection
[45, 46]
. The corneas of
these eyes have a hexagonal array of non-close-
packed subwavelength pillars forming a grating
that suppresses reflection of visible light (
Figure
12.1
). The dimensions and spacing of the periodic
pillar structures are smaller than the wavelength
of light where reflection is to be suppressed. As a
result, the effective refractive index of the coating
is reduced. This refractive index is the weighted
average by volume fraction of the refractive
index of the substrate and the space between
the pillars (typically air). The tapered structure
of subwavelength nipples can thus generate a
graded transition of refractive index, leading
to minimized reflection over a broad range of
wavelengths and angles of incidence
[45, 47, 48]
.
Various top-down technologies, such as pho-
tolithography
[49]
, electron-beam lithography
[50]
, nanoimprint lithographyy
[51, 52]
, and
interference lithography
[33, 45, 53, 54]
, have
been developed for fabricating moth-eye ARCs.
However, these techniques require sophisticated
equipment and are expensive to implement
[45, 55, 56]
. Bottom-up self-assembly and subse-
quent templating nanofabrication provide a
much simpler and cheaper alternative to com-
plex nanolithography in creating subwave-
length-structured moth-eye arrays
[2, 57-59]
.
